The present invention is directed, in general, to telecommunications and, more specifically, to a telephone line-powered power supply for ancillary equipment and a method of operating the same.
The addition of ancillary equipment to telephone systems has been increasing in both requirements and complexity primarily due to customer demands. Initially, the ancillary equipment included options such as automatic dialer circuits which typically provided memory for frequently called telephone numbers. The power supply requirements for these types of circuits are usually well within the capability of even plain old telephone systems (POTS) to provide.
Certain telephony applications, such as answering machines and speaker phones may require AC power to supply sufficient energy to the integrated circuits involved. In many earlier systems, the telephone circuits were entirely separate from the answering machine circuit. The two were joined only at the telephone line. Subsequent products used battery power as back-up for times when the AC power failed. Conventional line interface circuits provided little power to be used by any integrated circuit.
However, it is desirable in the U.S. and required in many foreign countries that if the AC power fails, the basic telephone functions of speech (speaking and hearing), network address, and alerting be maintained. This could technically be done by switching to an alternate set of circuits that are capable of providing these functions using only power from the telephone line. This approach, however, is obviously not economical. Also, conventional telephone line interface circuits which require at least a 600 ohm AC impedance do not allow much power for use in ancillary circuits.
In basic telephone line powered circuits, a common configuration is to use a diode bridge (polarity guard circuit) to guarantee voltage polarity, a switch hook (electronic or mechanical), a termination impedance as mentioned and a DC voltage-current characteristic circuit. This later circuit often consists of a bipolar transistor with an emitter degeneration resistor and a DC bias path for the base of the transistor. This DC bias path often includes the AC termination resistance of 600 to 1000 ohms, and a large capacitor to prevent attenuation of voice band signals. In this conventional circuit, starting with the required voltage-current system restriction of less than 6 volts across the telephone line with a loop current of 20 mA, we may calculate a DC power availability across the large capacitor of approximately 2.5 mA at 1.8 volts.
These values are insufficient to operate digital signal processor (DSP) or coder-decoder (CODEC) equipment. CODEC equipment converts voice signals from their analog form into digital signals acceptable to more modern digital PBXs and digital transmission systems. It then converts those digital signals back to analog so that the voice signal may be understood by the person who hears it. In some phone systems, the CODEC is in the PBX and shared by many analog phone extensions. In other phone systems, the CODEC is actually in the phone.
The maintaining of POTS capability encompasses several requirements. These include the setting of the specific DC voltage-current characteristic for the telephone line, as mentioned, while active. The AC line termination resistance of approximately 600 ohms and a bandpass frequency range of 300 to 3300 Hertz are typically required. Network addressing with dual tone multifrequency (DTMF) and pulse capability must be provided as well as alerting, the detecting of a xe2x80x9cringingxe2x80x9d signal and the sounding of an xe2x80x9calerterxe2x80x9d.
Speech capability for POTS includes full duplex coupling to a telephone line for transmitting and receiving voice signals. The amplification of microphone signals for transmission and the amplification of the receive signal for the handset speaker is necessary. In the U.S., the speech hybrid function is defined by average loudness templates referred to as TOLR, ROLR, and SOLR which are Transmit, Receive and Sidetone Objective Loudness Ratings respectively. Transmit and Receive also have normalized frequency templates as mentioned earlier. Sidetone is a part of the design of a telephone handset which allows one to hear their own voice while speaking. Too much sidetone result in an echo and too little renders the channel unerring. Therefore, controlling the amount of microphone signal that is audible in the handset speaker is critical.
Accordingly, what is needed in the art is an economical way for a POTS architecture, during active speech mode, to provide additional current and voltage for ancillary circuits.
To address the above-discussed deficiencies of the prior art, the present invention provides a telephone line-powered power supply, a method of operating the same and telephone line-powered ancillary equipment. In one embodiment, the power supply includes: (1) a transformer having a primary winding and a secondary winding and (2) an energy storage device (which may, but is not required to, be a capacitor) coupled to the primary winding, the primary winding and the energy storage device adapted to be coupled to, and provide a terminating resistance for, a telephone line. A node between the primary winding and the energy storage device provides DC power.
The present invention therefore introduces the broad concept of substituting a conventional terminating resistor with a power supply to increase the ability of a telephone line to provide power to ancillary equipment. In alternate embodiments, the power supply can include a transformer or be transformerless. xe2x80x9cAncillary equipmentxe2x80x9d is defined, for purposes of the present invention, as equipment that requires electric power. xe2x80x9cAncillary equipmentxe2x80x9d includes, but is not limited to, signal processing or other electronic circuitry located within a telephone and answering, caller identification or other equipment that a user may want to locate near a telephone. xe2x80x9cAncillary equipmentxe2x80x9d may be powered by the telephone line only as required or may be powered regardless of the availability of alternative power.
In one embodiment of the present invention, the power supply further includes a line transformer interface coupled to the secondary winding. The line transformer interface, which is not necessary to the present invention, provides a signal interface for signals derived from or supplied to the secondary winding. Ancillary equipment, such as signal processors, may employ a line transformer interface to communicate via the telephone line.
In one embodiment of the present invention, the power supply further includes a diode series-coupled to the primary winding. The diode can protect other components in the power supply from receiving discharges from the energy storage device.
In one embodiment of the present invention, the power supply further includes an activation switch couplable to the telephone line and functioning based on a hookstate. A xe2x80x9chookstatexe2x80x9d is literally the state of the telephone""s hook: xe2x80x9coff-hook or xe2x80x9con-hook.xe2x80x9d The ancillary equipment senses the voltage of an alternative source of electric power and enables the power supply with the activation switch when the alternative source of electric power is interrupted.
In one embodiment of the present invention, the power supply further includes a line voltage sense circuit couplable to the telephone line. The voltage circuit allows the voltage of the telephone line to be determined. The line voltage sense circuit is used by the ancillary equipment to bias and control other circuits necessary for proper telephone operation.
In one embodiment of the present invention, the power supply further includes a filter coupled to the secondary winding. The filter, while not necessary to the present invention, is desirable in some applications to smooth signals associated with the secondary winding.
In one embodiment of the present invention, the power supply further includes a power fail ringing circuit couplable to the telephone line. The power fail ringing circuit, if present, provides ringing signals in the event of an interruption of an alternative source of electric power.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.